An electric power steering apparatus that applies assist load to the steering apparatus of an automobile or a vehicle with turning effort of a motor applies the driving force of the motor to a steering shaft or a rack axis based on a transmission mechanism like gears or belts via a reduction gear. Such a conventional electric power steering apparatus carries out a feedback control of a motor current for accurately generating an assist torque (a steering auxiliary torque). The feedback control is for adjusting a motor application voltage so as to minimize a difference between a current control value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of a PWM (Pulse Width Modulation) control.
A general structure of an electric power steering apparatus will be explained with reference to FIG. 9. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of running wheels through a reduction gear 3, universal joints 4a and 4b, and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1. A motor 20 for assisting the steering force of the steering wheel 1 is connected to the shaft 2 through a clutch 21 and the reduction gear 3. A control unit 30 for controlling the power steering apparatus is supplied with power from a battery 14 through an ignition key 11. The control unit 30 calculates a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12. The control unit 30 then controls a current to be supplied to the motor 20 based on the calculated steering assist command value I. The clutch 21 is ON/OFF-controlled by the control unit 30, and is kept ON (connected) in an ordinary operation status. When the control unit 30 has decided that the power steering apparatus is in failure, and also when the power source (voltage Vb) of the battery 14 has been turned OFF with the ignition key 11, the clutch 21 is turned OFF (disconnected).
The control unit 30 is mainly composed of a CPU. FIG. 10 shows general functions to be executed based on a program inside the CPU. For example, a phase compensator 31 does not show a phase compensator as independent hardware, but shows a phase compensation function to be executed by the CPU.
Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then input is phase-compensated by the phase compensator 31 for increasing the stability of the steering system. The phase-compensated steering torque TA is inputted to a steering assist command value calculator 32. A vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value calculator 32. The steering assist command value calculator 32 calculates a steering assist command value I as a control target value of a current to be supplied to the motor 20, based on the input steering torque TA and the input vehicle speed V. The steering assist command value I is inputted to a subtractor 30A, and is also inputted to a differential compensator 34 of a feedforward system for increasing a response speed. A difference (I−i) calculated by the subtractor 30A is inputted to a proportional calculator 35, and is also inputted to an integration calculator 36 for improving the characteristic of a feedback system. Outputs from the differential compensator 34 and the integration compensator 36 are inputted to and added together by an adder 30B. A result of the addition by the adder 30B is obtained as a current control value E, and this is inputted to a motor driving circuit 37 as a motor driving signal. A motor current value i of the motor 20 is detected by a motor current detecting circuit 38, and this motor current value i is inputted to the subtractor 30A and is fed back.
An example of a structure of the motor driving circuit 37 will be explained with reference to FIG. 11. The motor driving circuit 37 comprises an FET gate driving circuit 371 for driving each gate of field-effect transistors (FETs) FET1 to FET4 based on the current control value E from the adder 30B, an H-bridge circuit composed of the FET1 to the FET4, and a step-up power source 372 for driving a high side of the FET1 and the FET2, respectively. The FET1 and the FET2 are ON/OFF-controlled by a PWM (Pulse Width Modulation) signal of a duty ratio D1 determined based on the current control value E, and the size of a current Ir that actually flows to the motor 20 is controlled. The FET3 and the FET4 are driven by a PWM signal of a duty ratio D2 defined by a predetermined linear functional expression (D2=a·D1+b, where a and b are constants) in an area where the duty ratio D1 is small. When and after the duty ratio D2 has also reached 100%, the FET3 and the FET4 are ON/OFF-controlled according to a rotation direction of the motor 20 determined by a sign of the PWM signal.
According to a widely-distributed hydraulic power steering apparatus, the apparatus has a characteristic that the friction of a cylinder section increases in proportion to a cylinder pressure P (a horizontal axis T represents a steering torque), as shown in FIG. 12. The apparatus has hysteresis because of the frictional characteristic. When a vehicle is cornering, for example, the hysteresis prevents the steering wheel from being suddenly returned by a self-aligning torque. This improves the steering of the driver. FIG. 13 shows this status. When the steering torque T has suddenly changed by A T, the cylinder pressure changes by P1 in the absence of hysteresis. However, in the presence of hysteresis, the cylinder pressure changes by P2(<P1). Therefore, in the presence of the hysteresis, it is possible to make smooth the change in the cylinder pressure P in relation to a change in the steering torque T. It has been known that the hysteresis width changes according to a size of friction. In the case of a rubber packing of a hydraulic cylinder, the rubber is compressed along an increase in the cylinder pressure. The hysteresis width increases based on an increase in Coulomb friction. It is important for the steering that the driver feels strong self-aligning torque at a neutral point, and does not feel so strong self-aligning torque when the vehicle is cornering. In this sense, it is ideal that, like in the hydraulic power steering apparatus, the friction (hysteresis) becomes small in an area of a small steering angle θ, and the friction (hysteresis) becomes large in an area of a large steering angle θ.
On the other hand, according to an electric power steering apparatus, the apparatus has constant friction independent of the assist torque T, as shown in FIG. 14. The electric power steering apparatus has a feature that it has a constant friction characteristic independent of steering force, as the Coulomb friction of the motor mainly rules out. Thus, the hysteresis has a characteristic that it has a constant width as shown in FIG. 13. However, the hysteresis width is narrower than the hysteresis width of the hydraulic power steering apparatus during its high-torque time. Therefore, in the electric power steering apparatus, the friction is compensated for in the area of a small steering torque T by attaching importance to the friction characteristic in this area. According to this compensation, however, the friction becomes smaller in an area where the steering torque T is large, as shown in FIG. 15. As a result, the stable feeling of steering is lost when the steering torque T is large when the vehicle is cornering.
As a control unit that solves the above problems, there is one example disclosed in Japanese Patent Application Laid-open No. 9-156526 A. According to this, a vehicle steering control unit has a steering torque detector for detecting a steering torque, and this control unit controls the assist volume of an electric power assisting unit, based on a detection signal outputted from this steering torque detector. In this vehicle steering control unit, there is provided an adjuster for giving the hysteresis to the detection signal of the steering torque detector.
With the provision of the adjuster, it is possible to give the hysteresis to the detection signal of the steering torque detector. Therefore, it is possible to change the hysteresis characteristic of the operating power assisting unit according to the steering status, based on the detection signal of the steering torque. As a result, it is possible to optimize the torque assist volume. However, according to this conventional unit, there remains a feeling of intermittence in the steering operation, and the torque control system is unstable. Thus, there has been a problem in that the conventional unit leads to a cost increase because of the need for a provision of new hardware structure.
Further, the present applicant has disclosed a device in Japanese Patent Application Laid-open No. 2000-95131 A. According to this device, a negative differential gain is applied when the steering wheel returns, thereby to prevent a sudden reduction in the assist volume. A positive differential gain is applied when the steering wheel is turned. As a result, a large hysteresis characteristic is given in a high-torque area, and a small hysteresis characteristic is given in a low-torque area near the neutral point. However, according to this device, there is a risk of the occurrence of an unnatural feeling of steering, when the negative and positive differential gains are too different in the changeover between the negative and positive differential gains based on a steering pattern of the returning and turning of the steering wheel.
Further, Japanese Patent Application Laid-open No. 10-291481 A discloses a device capable of obtaining a comfortable steeling feeling regardless of a running speed and a steering angle of the steering wheel. However, the importance is placed on only the stability of the control system, and therefore, this device has a problem in the responsiveness of the assist torque.
The present invention has been made in the light of the above situations. It is an object of the present invention to provide a control unit for an electric power steering apparatus capable of obtaining continuous, stable and comfortable feeling of steering, for improved steering performance of the steering wheel, based on a provision of a continuous hysteresis characteristic in an adjustable width to the electric power steering apparatus using a low-cost structure on software.